The invention relates to an interface device for interfacing a network station to a physical network medium. The network may be a ring, star, bus or other type set-up, notably in an LAN-environment, and represented by a coax cable, twisted wire pair, optical fibre, or other. The station in particular may be an industrial station, for example, a sensor, drive unit, machine, material management unit, computer, display screen station etc.
For fast and comparatively simple message traffic between the various stations and the International Standards Organisation has drafted recommendations in the Open Systems Interconnection (OSI) model. This OSI model provides recommendations for a structure consisting of seven layers. For example, layer 1 relates to the physical communication between the stations and layer 2 relates to the Medium Access Control (MAC) and the Logical Link Control (LLC); these recommendations define the accessing of the network for the transmission of messages. For the present invention layers 3 to 6 are not practically important, layer 7 concerns the application software for controlling the functions of the relevant stations.
The OSI recommendations are standardized in various protocols for use in a LAN. Two examples are MAP (Manufacturing Automation Protocol) and Ethernet. MAP is a token passing protocol whose layer 1 and Medium Access Control (MAC) (the lower half of layer 2) are described in IEEE Std. 802.4. Ethernet is based on a Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) system. For Ethernet and similar protocols, for example TOP (Technical and Office Protocol), the physical execution and the MAC are described in IEEE Std. 802.3. MAP and Ethernet use the same Logical Link Control (LLC) (the upper half of layer 2) as described in IEEE Std. 802.2.
In layer 1 of the OSI recommendations the signals are applied to the physical medium via modulator, as a bit stream that is demodulated in a receiving station. In the case of MAP, bit rates are from 1.5 to 10 Mbit/s Ethernet from 5 to 10 Mbit/s. Various modulation techniques can be employed, such as amplitude modulation techniques (for example non return to zoro, NRZ) or frequency modulation techniques (for example Frequency Shift Keying, FSK). At present phase-continuous FSK modulators NE 5080/5081 by Signetic comply with IEEE Std. 802.4, and also the phase-coherent carrier band modem (CBM) by Motorola. Known hardware for implementing layer 2 is still not cost-effective for simple applications, for example MC 68824 (by Motorola), that is compatible to the CMB modem. Using this unit, the LLC operations and those of the higher OSI layers must be accommodated in additional station hardware (See also: Computer Design, Feb. 1, 1986, pages 69-73, and IEEE Micro, June 1986, pages 15-25).